Melt out metal cores of complex shapes are made for use as cores in subsequently molded plastic components. The cores are made of metal alloy or other suitable material having a low melting temperature. Cores are placed in molds for making undercut hollow plastic components and then subsequently removed from the plastic components by melting the cores and leaving the undercut or hollow one piece plastic components. The melting temperature of the solidified metal alloy or other material is lower than that of the plastic component. In other embodiments metal alloys with low melting temperatures are used for encapsulating components such as turbine blades so they may be held for machining in other finishing steps.
In U.S. Pat. No. 4,958,675 a metal casting process is disclosed wherein the injection cylinder is filled with molten metal alloy from the tank through a valve port in the injection passageway leading to the injection cylinder by raising the piston in the cylinder. In U.S. Pat. No. 4,991,641 a similar apparatus is disclosed wherein two valves in line are positioned within the tank that forms a single assembly for ease of handling.
In our co-pending application Ser. No. 578,835 is disclosed an injection nozzle for joining a die to an injection passageway. The nozzle is flexible and also has a check valve incorporated therein so that the liquid metal flow is stopped except when the nozzle interfaces with the die.
When low temperature melt out parts or encapsulations are made, it is necessary to fill the die slowly under substantially no pressure to ensure that a uniform density and fine grain structure is achieved. Such a process is quite different from that of die casting where injection pressures are generally in the range of about 800 to 4,000 pounds per square inch and the time of injection is in the order of 30 to 40 milliseconds. In such an operation hot metals are injected at high velocity and with a turbulent flow into a die through a narrow gate. Air or other gases can sometimes become entrapped and pressures build up in the cylinder and injection passageway. These high speed injection processes generally include runners leading into the die and the unsolidified metal drains back after the casting process.
Melt out metal parts are generally made from metal alloys having a melting temperature below 350.degree. C. High pressure die casting generally uses metals with much higher melting temperatures, and such pressures are not appropriate for making melt out metal parts or encapsulations because the desired dimension tolerances and freedom from voids cannot be achieved. Castings of melt out metal alloys are generally produced by allowing liquid metal alloy to flow into a die under substantially no pressure. After the die is full, a small pressure generally in the order of 30 to 50 pounds per square inch is built up in the die during the cooling stage. The filling and cooling time can vary from about 3 to 30 seconds depending upon the capacity of the die.